Polarization Observables T and F in Single π 0 and η -Photoproduction off Quasi-Free Nucleons Thomas Strub, A2 Collaboration University of Basel 28th August 2014, PANIC 2014 A 2
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Outline 1 Motivation 2 Experimental Setup 3 Polarization Observables 4 Analysis Methods 5 Selected Results 6 Conclusion Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Motivation Problems on experimental side ◮ Nucleons’ excitation spectrum is a complicated overlap of many short lived, broad resonances ◮ Cannot be understood from differential cross sections alone Problems on theory side ◮ QM predicts more states than observed (missing resonances) ◮ Perturbative QCD cannot be applied in this energy region ◮ Lattice gauge QCD cannot (yet) reproduce all desired properties Mass/(MeV/c 2 ) N(I=1/2) ∆ (I=3/2) exp QM QM exp H 3,11 (2420) 2400 F 37 (2390) G 19 (2250) D 35 (2350) H 19 (2220) H 39 (2300) D 15 (2200) G 17 (2190) 2200 P 11 (2100) S 31 (2150) S 11 (2090) F 35 (2000) D 13 (2080) F 37 (1950) D 33 (1940) F 15 (2000) 2000 D 35 (1930) F 17 (1990) P 33 (1920) P 13 (1900) P 31 (1910) P 13 (1720) F 35 (1905) P 11 (1710) 1800 S 31 (1900) D 13 (1700) P 31 (1750) F 15 (1680) D 33 (1700) D 15 (1675) S 11 (1650) S 31 (1620) 1600 P 33 (1600) S 11 (1535) D 13 (1520) P 11 (1440) 1400 P 33 (1232) 1200 1000 P 11 (939) Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Motivation Solution ◮ Effective quark models ◮ Experiment delivers observables to fix the models parameters via PWA Polarization observables from meson photoproduction ◮ Probing spin degrees of freedom ◮ Unique PWA solution = ⇒ Need 8 (of 16) carefully chosen observables for complete experiment ◮ Include angular momentum L ≤ 3, i.e., S, P, D, F-wave = ⇒ Need full angular coverage, high precision measurements Proton and neutron channel ◮ Probe isospin degree of freedom ◮ Isospin decomposition into A V 3 , A IV , A IS for π photoproduction � � � � 1 2 2 1 3 A V 3 + � A IV − A IS � 3 A V 3 + � A IV − A IS � A ( γ p → π + n ) = − A ( γ p → π 0 p ) = + 3 3 � � � � 1 2 2 1 3 A V 3 − � A IV + A IS � 3 A V 3 + � A IV + A IS � A ( γ n → π 0 n ) = + A ( γ n → π − p ) = + 3 3 = ⇒ At least one measurement off the neutron needed. Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Motivation Mass [ MeV/c 2 ] Special case η photoproduction N(I=1/2) � (I=3/2) D 13 (1700) D 33 (1700) F 15 (1680) ◮ Isospin I = I z = 0. D 15 (1675) S 11 (1650) S 31 (1620) 1600 P 33 (1600) ◮ No isospin changing current ( A V 3 = 0) S 11 (1535) D 13 (1520) P 11 (1440) A ( γ p → η p ) = A IS + A IV 1400 A ( γ n → η n ) = A IS − A IV P 33 (1232) 1200 ⇒ only N ∗ ( I = 1 � = 2 ) resonances contribute ◮ Recent results show a narrow structure Notation : 1000 L 2I2J ; L=0(S),1(P),2(D),... P 11 (939) � � � � � around 1670 MeV Photoproduction off the neutron ◮ Neutron bound in nucleus = ⇒ quasi free neutron ◮ Correct treatment of Fermi motion ◮ Comparison of free and quasi free proton data (can differ due to FSI) Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Experimental Setup Experimental Setup Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion MAinzer MIcrotron High quality electron beam ◮ Energy up to 1.5 GeV ◮ Intensity up to 100 µ A ◮ Polarization ≈ 80 % Bremsstrahlung photons ◮ 1/ E γ distribution ◮ Photon polarization: Olsen maximum function 1 Polarization degree 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 200 400 600 800 1000 1200 1400 Photon Energy Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Crystal Ball/TAPS @ MAMI CB ◮ PID ◮ MPWC ◮ NaI crystals TAPS ◮ BaF2/PWO crystals ◮ Veto wall = ⇒ Almost 4 π acceptance Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Polarization Observables Polarization Observables Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion General Formula General decomposition of d σ into 16 polarization observables reads Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Definition of T and F ◮ Target asymmetry T P T = P T ⇒ Transversally polarized target � = x � = 0 ◮ Double polarization observable F P T = P T ⇒ Transversally polarized target � = x � = 0 P γ = P γ Circularly polarized photon beam � c � = 0 P R = 0 (unpolarized recoil), � P γ = P γ ◮ For � c (circular photon polarization) P T = P T and � x (transverse target polarization) the general decomposition reduces to P T , 0 ) = 1 � � d σ ( � P γ , � 1 + TP T c P T 2 d σ 0 y + FP γ x = 1 � � 1 + T | P T | cos ( φ ′ ) + F | P γ || P T | cos ( φ ) 2 d σ 0 . ◮ Observables T and F manifest themselves by a cosine- φ ( ′ ) -modulated unpolarized cross-section Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Definition of T and F (experimental approach) T and F can be rewritten as d σ ↑ ( φ ′ ) − d σ ↓ ( φ ′ ) 1 T cos ( φ ′ ) = d σ ↑ ( φ ′ ) + d σ ↓ ( φ ′ ) , P T P γ where ( ↑ , ↓ ) denotes the target polarization state, d σ − ( φ ) − d σ + ( φ ) 1 F cos ( φ ) = d σ − ( φ ) + d σ + ( φ ) , P T P γ where (+ , − ) denotes the photon helicity state. ◮ φ = Angle between target Here, F = F ( E , θ ) , T = T ( E , θ ) , P T = P T ( t ) polarization vector and and P γ = P γ ( E γ , P B ( t )) production plane ◮ φ ′ = Angle between target ◮ Symmetric contributions cancel in the polarization vector and numerator normal to production plane ◮ Denominator equals unpolarized d σ Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Methods to extract T and F Polarized target: D-butanol ( C 4 D 9 OD ) ◮ Only deuterons are polarized ◮ Carbon/oxygen contribution vanish in numerator Two methods can be used: ◮ 1. Normalize with deuterium target DB ( φ ) − d σ + d σ − 1 DB ( φ ) A cos ( φ ) = P T P γ d σ − D ( φ ) + d σ + D ( φ ) = ⇒ Needs flux and efficiency correction of count rates. ◮ 2. Normalize with D-butanol target dN − DB ( φ ) − dN + DB ( φ ) 1 A cos ( φ ) = DB ( φ ) · d dN − DB ( φ ) + dN + P T P γ = ⇒ No need for flux and efficiency correction, but dilution factor d , i.e., d = 1 + d σ 0 C d σ 0 DB Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Analysis Methods Analysis Methods Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Event selection ◮ Event selection ◮ Full exclusive on proton (neutron as spectator) → π 0 + p ( n ) γ + d − → 2 γ + p ( n ) 2 neutral, 1 charged − γ + d − → η + p ( n ) → 2 γ + p ( n ) 2 neutral, 1 charged − → 3 π 0 + p ( n ) γ + d − → η + p ( n ) − → 6 γ + p ( n ) 6 neutral, 1 charged − ◮ Full exclusive on neutron (proton as spectator) → π 0 + n ( p ) γ + d − → 2 γ + n ( p ) 3 neutral, 0 charged − γ + d − → η + n ( p ) → 2 γ + n ( p ) 3 neutral, 0 charged − → 3 π 0 + p ( n ) γ + d − → η + n ( p ) − → 6 γ + n ( p ) 7 neutral, 0 charged − ◮ Determination of the neutron candidate by χ 2 -test. ◮ Invariant mass cut on all 3 π 0 from η → 6 γ decay. Polarization Observables T and F Thomas Strub, A2 Collaboration
Outline Motivation Experimental Setup Polarization Observables Analysis Methods Selected Results Conclusion Applied Cuts All cuts are determined from LD 2 target for all θ and energy bins. ◮ Coplanarity cut ∆ φ = 180 ◦ − | φ meson − φ recoil | ◮ Invariant mass cut meson | − m theo. ∆ m meson = | P µ meson ◮ Missing mass cut meson | − m theo ∆ MM = | P µ γ + P µ nucleon − P µ nucleon Polarization Observables T and F Thomas Strub, A2 Collaboration
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